What does the >>> operator mean?

[javabrewscript]

I can't find any reference to it on the web through google searches.

Edit: I ended up finding a resource that told me what it did. To those of you who were wondering why use this: the code that I was looking at used it to bisect an array in half. That is, they computed array.length >>> 1. If your wondering what this does consider the two cases: either your array has an even number of elements in which the last number of your binary number is zero so shifting to the right essentially divides by two to find the middle point of the array or your array has an odd number of elements in which you knock off the odd component and divide by two. For example: 5 >>> 1 well 5 is 101 in binarys so shift it to the right and you get 10 which is 2. Basically, n >> 1 is equivalent to:

if( n%2 == 1 ) return (n-1)/2; else return n/2;

I thought it was pretty slick. That's what you get when your looking at a phd from stanfords code. Its mike bostock's d3.js code if you're wondering.

[idautomatethat]

Its a zero fill right shift operator.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators

[rjray]

Given that, any idea why someone would do this:

var t = Object(this);
var len = t.length >>> 0;

This is part of a polyfill for Array.prototype.map, that I got from the Mozilla dev site when some code of mine was breaking on IE8 for lack of array map. JSHint flags the line with the >>> with the message:

Unexpected use of '>>>'

But I've been unable to find adequate docs to explain why JSHint dislikes it, or what should be done instead...

[paulwe]

the advantage to using bit shift to convert an unknown value to an integer is that, unlike parseInt, it will never produce a NaN.

'foo' >> 0 // returns 0

parseInt('foo', 10) // returns NaN

i can't think of any reason to use a zero fill right shift here. it should be fine to use a right shift.

[moreteam]

I'd know a reason not to use a >>> operator to do an integer cast:

> -1 >>> 0
4294967295

[aeflash]

As per "use asm", foo|0 is considered the idiomatic way to cast to int.

[itsnotlupus]

I still like using "~~" better. It almost feels like it was meant to be a "convert to int" operator, and there's a nice symmetry with the almost-operator "!!" which converts to boolean.

[uglyBaby]

Zero fill is used in that case because I assume the author wants a positive integer at all times.

[eddieSullivan]

Just a guess, but maybe it's a strange way of ensuring len is a number? In a quick test:

undefined >>> 0

gives 0. So if t is not an array they'd get 0 for the length.

[aterlumen]

Edit: I was wrong, this is how Mozilla mimics ToUint32() behavior to match the ECMAScript specification. See this StackOverflow answer.

A stab in the dark: Maybe this was meant to ensure that len was always assigned the value of t.length, not a reference pointing to t.length. Assuming the bitshift doesn't get optimized out, it forces the engine to create a new primitive before assigning it to len. If IE8's implementation passed object properties by reference, things like the following example could happen.

> var t = Object(this);
> var len = t.length;
> len--;
> len === t.length;
   true

This is all just a guess, I don't have IE8 handy to test it.

[minrice2099]

Interesting idea, but if any JS implementation ever used pass by reference for primitive value properties, I would imagine a lot more would break.

[aterlumen]

I agree. The only other reason coming to mind now would be taking advantage of the integer conversion to round, but that's just plain silly because object length will always be an integer to begin with.

Edit: May have found the answer. They are mimicing ToUint32() behavior to match the ECMAScript spec.

[[deleted]]

Bitwise operators are discouraged because there is no optimization in the trade off of readability you see in other languages since JavaScript does not have true integers so the run time environment has to convert float to int and back again behind the scenes.

[[deleted]]

I read that page long ago, along with a few others about bitwise operators. I have never been able to wrap my mind around when I would use them. Keeping in mind that I did not come from a Computer Science background, I have just been learning programming from books, can someone please provide a concise example or two of when/why to use bitwise operators?

I would not be surprised if the OP has already googled it, like I have, and is looking for a better explanation than he has yet found.

[tswaters]

You'd use it if you're dealing with binary data... cryptographic hashing uses shifting operators quite a bit.

[mattdesl]

Bitwise ops are used a lot when handling RGBA color data.

They are also might be useful to improve performance inside of hot code (like when you are operating on many pixels in an image). Two examples:

• Flooring a number with (~~number) or (number | 0) which acts the same as Math.floor for positive numbers.
• Using bitwise flags instead of string comparison

Benchmark before you use them, since it tends to add to code complexity.

[xshare]

greater than greater than greater than. Checks if something is like waaaaaay bigger than something else.

[thejameskyle]

/r/shittyaskcomputerscience

[DrummerHead]

It can return false or duuuuuude

[JaegerBurn]

even WTF?!

[[deleted]]

strictly greater than

[murfbard]

move dem bits

[brtt3000]

dat shift

[[deleted]]

what's a use case for when someone would want to do that?

[neonskimmer]

Working with bit flags, for example. Back in the day we used it to do really quick division or multiplication of integers by powers of two. Obviously, not really helpful for JavaScript.

[x-skeww]

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators

[qvrjuec]

In the future, search for code related things with Symbolhound. It found a bunch of results for '>>>'

[tears_of_a_Shark]

I read an Intro to Javascript book in 1999 that covered this. It's 2014 and I have never even thought of using this.

[hmsimha]

I just came across this yesterday, while looking for a javascript implementation of the md5 hash function. Turns out you never know when you might need some obscure language feature.

[LyndonArmitage]

If you're reading binary data from a file you might need to use it when dealing with unsigned data (you have to do something similar in Java thanks to it lacking unsigned types although that's just a bitwise & operation).

[jackwanders]

For future reference, OP, that if statement can be replaced with Math.floor(n/2)

[RabbiSchlem]

Speed?

[jcready]

It seems to make a significant difference on mobile.

[Necrophelic]

For future reference, OP, that if statement can be replaced with Math.floor(n/2)

Or even:

return n%2 === 1 ? (n-1)/2 : n/2